This invention relates to a method for the production of a low density polyethylene by a slurry copolymerization of ethylene and an .alpha.-olefin.
It is well-known in the art that polyethylene is produced by effecting a slurry polymerization in the presence of such hydrocarbons as hexane and heptane at temperatures of not more than 100.degree. C., at which the resulting polymer is not dissolved, or by effecting a solution polymerization at temperatures of more than 100.degree. C., usually 130.degree. C. or more, at which the resulting polymer is dissolved. Hereinafter, the former is referred to as a slurry process and the latter as a solution process.
Various grades of polyethylene have been produced in a commercial scale by controlling the molecular weight and density of a polymer in accordance with its uses. It has, however, been impossible to provide polyethylene covering various grades in a wide range by the slurry process alone or the solution process alone.
As reported in, for example, Chemical Economy & Engineering Review, 7, 24-40 (1975), the above two processes are used properly depending on the desired grades.
The relation between grades which are defined by the density and melt index of polyethylene and the production process therefor is summarized from the abovementioned literature as shown in Table 1.
Table 1 ______________________________________ Melt Index More than Less than Density, g/ml 1.0 1.0-0.1 0.1 ______________________________________ Slurry Slurry process process Slurry 0.945- 0.970 process Solution Solution process process Less than Solution Solution Impossible 0.945 process process to produce ______________________________________
It is well-known that melt index of polyethylene can be controlled depending on polymerization conditions and amount of a molecular weight modifier (for example, hydrogen) used in the polymerization, and that density of polyethylene can be controlled by copolymerizing ethylene with an .alpha.-olefin.
In the production of a high density polyethylene (higher than 0.945 g/ml) by the slurry process, the resulting polymer is insoluble in a polymerization solvent and therefore, the viscosity of a polymerization system is not directly affected by the molecular weight of a polymer. Accordingly, polymers of from higher molecular weight (lower melt index) to lower molecular weight (higher melt index) can be obtained. However, in the event of producing a low density polyethylene by the slurry process through a copolymerization of ethylene with an .alpha.-olefin, there are many troubles that the viscosity of a polymerization system increases due to the formation of polymers soluble in the polymerization solvent and the swelling of the resulting polymer results in the lowering of bulk density and the stickiness of filter cakes.
Under such troubles, it is very difficult to conduct the production in commercial scale conveniently. Therefore, the grades of polyethylene which can be produced by the conventional slurry process are limited to those having a density of 0.945-0.970 g/ml and a melt index of 0.01-40. In effect, the so-called, Ziegler method's polyethylene which is mainly produced by the slurry process is named a high density polyethylene.
In the refining of a polymer containing slurry obtained in the slurry process with use of the known catalyst consisting of titanium trichloride and an organoaluminum compound, the after-treatment of decomposing the catalyst and then removing it by washing requires a long time, and during the time the viscosity of the polymer slurry increases and the bulk density of the polymer lowers. Thus, the use of supported catalysts has been proposed to obtain the polymer slurry of a good property, having no need of the complicated after-treatment. For example, US Pat. No. 3,888,835 discloses a process for the polymerization of ethylene, in which a high activity catalyst having a titanium compound supported with a magnesium compound is used and such a low-boiling hydrocarbon as butane is used as a diluent for polymerization.
However, when this process applies to the production of the low density polyethylene having a density of 0.935 g/ml or less through copolymerization of ethylene with an .alpha.-olefin, the resulting polymer is lowered in bulk density and it is difficult to obtain the polymer slurry of a good property. The lowering of bulk density brings about troubles in handling of the slurry, such as stirring, transferring, filtrating, drying and storing. For example, if the resulting polymer is reduced in bulk density in the polymerization step, the formation of bulky polymers results in insufficient agitation and accordingly, in lowering of the diffusion velocity of ethylene so that the apparent polymerization velocity is extremely reduced in spite of the catalytic activity being still sustained. Also, since the bulky polymers adsorb the solvent used, the pump conveyance and filtration of the slurry can not efficiently be conducted. For avoiding such troubles, the polymer slurry must be diluted with a large amount of solvent and as a result, contains such as a polymerization vessel and a storing vessel and installations for the recovery step of the solvent used and for the refining step are run on an extensive scale which is extremely disadvantageous economically.